Convert *.FBX to Solid for Physics Sim
The FBX should presumably include a mesh when it is imported. Most likely if it came from Mixamo it also includes an armature, and the mesh will be parented to that armature.

Make sure you select the mesh, not the armature, and then you should be able to add physics properties to the mesh. 

In this case you want to go to the Physics tab of the properties editor and add collision to the object. This will tell any cloth objects that this object should be treated as an obstacle.